Estrogen, androgen and progestogen uptake is being examined in the brain and pituitary of male and female rats. Nuclear localization of the intravenously-administered, tritiated hormone is visualized via dry-autoradiography. Pituitary autoradiogram sections are, in addition, immunocytochemically stained with antiserum to various adenohypophyseal hormones in order to allow identification of cell types. Silver grains are counted over nuclei of each adenohypophyseal cell type and over nuclei of given hypothalamic and amygdaloid regions. Animals are examined following various intervals of castration, thyroidectomy and adrenalectomy or following hormonal pretreatment. One day after castration estrogen uptake in immunocytochemically characterized pituitary cell types is as follows: PRL equals GH greater than LH beta equals FSH beta greater than beta endorphin greater than TSH beta. Pituitaries from males tend to concentrate slightly more radioactivity. By two weeks after castration gonadotropes concentrate considerably more estrogen than lactotropes and somatotropes while thyrotropes and corticotropes concentrate only very small quantities. Progesterone pretreatment appears to bring out striking sex differences in the estrogen uptake by the adenohypophysis. These autoradiographic and immunocytochemical data are being correlated with parallel ultrastructural studies.